Cleaned up zone-contents.c.

src/knot/zone/zone-load.c

src/libknot/nameserver/name-server.c

src/libknot/updates/ddns.c

src/libknot/updates/xfr-in.c

src/libknot/zone/zone-contents.c

src/libknot/zone/zone-contents.h

src/libknot/zone/zone.c

src/libknot/zone/zone.h